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Advice would be welcome on whether or not we should go down the heat pump route.

We are in the process of having a timber framed single storey annexe built. Floor area is ca. 90 sqm and will contain two bedrooms, a shower room and a large living area. It will be well insulated. We currently have 3.84kWp PV and will be installing an extra 5.88kWp bringing the total up to 9.72kWp. We originally intended having a GSHP with UFH but on calculating the size for a pump we were told that the heat demand is too low to justify having a GSHP and that a pump would be inefficient due to cycling. We turned down the suggestion of heating both the new and old building with a single GSHP for many reasons. This same company at our request considered the alternative of using an ASHP. Their calculation for the building heat demand with an external temperature of -1.6 C and 20 c inside was 2.3kW. They specified an ASHP of 8kW a NIBE F2040-8, which happens to be the smallest ASHP that the company lists. To my untrained eye this would be grossly over powered.

Thinking, perhaps wrongly, on this miss match I stumbled on a post on this forum by GarethC on 1st Feb 2016 that said, ĎAir source heat pumps are expensive (capital wise) and you will be spending a lot of money to provide very little heating if you have modern insulation levels. One problem with using an air source heat pump with a new build, compared to an existing, relatively poorly insulated building, is that new builds only need to be heated when it's really cold outside, and when it's really cold, the performance of a heat pump dropsí. This lead me to thinking that what would really be appropriate would be a thermal store with a 3kW immersion heater partly and at times completely powered by the PV. The TS could be connected to our existing oil CH boiler in the existing house and this and Economy 7 would be utilised in deep winter.

Is this a goer and if so would we be advised to have a separate DHW cylinder as we would not wish to power up the TS during the summer months, not that we would not have sufficient PV generation, but due to the general heat escaping from the TS.

There is probably a lot I donít understand on these issues but I am most willing, with your help, to be enlightened.

Theoretically only needing very little heating and the real world are very different. Unless you have sufficient ( 300mm+) levels of insulation and an air tight shell you will need heating, although it may be able to incorporate some passive solar elements. It is worth spending time doing your own research and calculations and understanding your realistic heating requirements rather than relying on a company who are selling you the equipment and just using generic software to do the calculations. You don't mention the existing structure or heating of the rest of the dwelling but a GSHP has a high capitol cost, an ASHP with ducted air much lower. Kensa do some small GSHPs.

Echoing Richard's thoughts, why not extend the existing oil heating system? But if you're still interested in taking the ASHP route..... You do not mention whether it would be an air-air HP or air-water, nor do you say which part of the UK you are based in. I have purchased both systems but have not yet installed them. One is an air-con, Chinese self-install HP that should produce 3 kw, the other is an Atlantic 6 kw air-to-refrigerant gas HP with heat exchanger to run DHW and UFH. Some years ago, before buying these units, I looked into adding an air-to-water heat pump which would not replace our oil boiler but would run in tandem with it. I did not pursue this any further because of the high initial start-up current required; we are restricted to max 30 amps.

You too could have a hybrid oil/HP system running the whole house. Navitron sell the equipment needed. The Panasonic units are high-end, expensive, but the Orilux ones are relatively cheap. No doubt, Navitron can also advise you how to go about installing or having a dual system installed.

BTW, was out shopping in the very cold weather at the weekend, real feel outside, -11 degrees, but I felt pleasantly warm underneath a ceiling-mounted Mitsubishi HP unit pumping out plenty of heat.

You have inidcated the amount of power required when it is -1.6C outside and 20C inside as 2.3kW but not indicated how much energy is actually required. If you can find a localish weather station you should be able to download degree-day information from the degree-days website (can't remember the site address at the moment), this would give you the total number of degree days you would require heating and from that the total amount of heat you would require. You might have to get some help to determine a reasonable base figure given the high level of insulation.

At only 2.3kW when it is cold outside, the capital cost of installing a wet system may easily exceed the cost of just using electricity directly to provide the heating. Given that you are intending to have a largish PV installation this would quite likely cover the heating required in spring/autumn assuming that the annexe retains the heat overnight reliably. In winter when the PV may produce very little output and the heating might be most needed you might have to rely on mains electric but even then assuming a worst case you would be below 50kWh per day which is £7.50 assuming 15p/kWh. You should be able to get a good idea of the PV production on a monthly basis.

Say you get a figure of 2000 degree days, which is 48000 degree hours and assuming 100Wh per degree hours is needed, that comes out at 4800 kWh pa at 15p/kWh £720 per year based on mains electric prices. If you can get 90% of the power from your own PV that would be under £100 per year heating.

You do need to put your own figures into a set of calculations like those above and work out what is realistic in terms of the heating required (probably the hardest part) and from that you can then get an idea of costs for each option.

Thanks for your replies and suggestions and I do apologize for not getting back sooner but I was whisked off to hospital for a few days, but allís well now.

It was pointed out that some vital statistics were overlooked by me and I can confirm that we intend having a wet underfloor heating system and are located a mile and a half from the west Dorset coast at 400 feet above sea level. Additionally the general insulation levels of the building are that the flat roof will have 300mm of Earthwool and the timber frame walls will have two 100mm layers with a separating cavity filled with another 50mm of Earthwool or hemp bats.

Titanís ĎTheoretically only needing very little heating and the real world are very differentí. A recent study apparently showed that new buildings very seldom live up to their designed insulation levels. As building progresses on the annexe I am beginning to understand why this is so for insulation would have been left out entirely in certain sections if I had not been on the ball and the general lack of attention to detail or even understanding the principals of good practice come as a shock.

Countrypaul suggested that I carry out my own heating needs calculation but this was done by the building designer. His findings were that annually it would require just 1200 kWh. Unfortunately he will not allow his results to be used by a third party for fear of reprisals in case of problems! Two heating companies that I have sought quotes from for the installation of an ASHP and UFH both arrived at markedly different heating demand of 5283 and 4920 kWh from our building designer. Why a discrepancy here? Perhaps the designer was over ambitious in his own design or the two companies were fitting their results to the size of heat pumps available. Assuming that the actual demand is somewhere between 1200kWh and 5000kWh per year then Countrypaul has done the calculations and reasoning for me. Iíve almost decided to install a TS with inputs for a feed from the oil boiler, a 3 kW immersion heater and connections for a heat pump. The heat pump connections would be kept in reserve just in case it proves that the PV canít cope and too much reliance is made of the oil boiler.

If you can recommend a thermal store that would fit this bill perhaps you can let me know.

I have a copper thermal store supplied by Newark via a third party. Due to the tight space I had available (dormer bungalow) I found most of the off the shelf ones were either much smaller than I wanted or too tall if big enough. The TS is 430L, with direct connections for WBS, and ASHP, a solar coil, 3 x 3 KW immersion heaters, the output has 3 connections for the DHW, one at the top for the hottest water, one lower down and one near the bottom for cold water in. The idea being that rather than mix the Hot water with cold to supply DHW it is mixed with the warm water from the lower connection, thereby reducing the amount of Hot water used and minimising the likelyhood of a tank of lukewarm water (how this will work in practice remains to be seen since we have not yet moved in). The 3 immersions was to allow us to use immersion heaters for the central heating (UFH) in the eventality of ASHP breakdown etc. - in practiec they have heated the house over winter (E7) since we don't yet have the ASHP and needed to ensure the UFH did not freeze (building was not weathertight).

If you are truly at 1200KWh per year for heating (did the 2 companies include DHW by any chance?) then if this could be run off E7 using immersions you would be looking at under £100/year meaning you might never recover the cost of installing a heat pump! Whilst most of the heat is probably required in winter when the PV is least productive, with 9.7 KWp of PV you might find that the amount of E7 used is significantly less than £100. Of course if the calculations are wrong and you do need 5000KWh, then that is still only £400 per year on E7 assuming no contribution from the PV, again it might make the heat pump installation appear unnecessary. Since you appear to have an option to get some heat from the oil boiler - assuming it has the capacity, that would work out at around 5-6p/KWh (assuming 50p/L kero) so possibly less expensive than E7 since no capital cost. For 120KWh that would be around 120L of Kero less losses.

It might even be worth omitting the thermal store and if additional heat is required in very cold spells, simply using an oil filled radiator (s) run directly off the mains. This would save significantly on the capital cost (TS and installation) whilst the running cost might be high they would likely be for only short periods. It would also be possible to run the oil filled radiator off the PV surplus since it is a purely resistive load which might also significantly cut down the amount of mains electric actually needed.

As heating loads drop due to better insulation, the actual means of heating need to be completely revised as the capital costs can completely outweigh the running costs resulting in an overly complex solution to something that can be solved relatively simply.

I presume the old part has rads and the new part underfloor. As i see it the problem is the rads are designed to run a 60C and the underfloor at say 30C and to different time regimes. So you need a tank for underfloor which can be heated by the oil boiler if necessary. This can be quite small. It could have tappings for a HP to future proof one for the future.

For DHW you could have another tank (which you may already have ) which in emergencies could be heated by oil boiler but would generally be heated by solar. Not knowing the number of people and their washing habits it is hard to say but i would put the partial solar days at say 20days mainly in Dec/Jan

The 1200 kwh annual heating for the new bit alone i feel is in the correct ball park. Average 12kwh for 100 days heating. If you run the house cooler than 21C and you will very rarely get minus temps in that part of the world next to the sea and the southerly breezes then the heating load will be less in my opinion. In your architects cals he should have included passive solar as relevant but not the effect of occupation which adds heat (unless someone starts opening windows).

I look on the heating load as a kind of triangle where the base is the 100 days then if the vertical ht is kwh/day then a peak of 24kwh/day(the coldest day) gives the 1200kwh/yr. Sounds about right to me.

The question of heat pump i feel is irrelevant as it is just another type of heat source, take it or leave it. Take it if you want to reduce oil and be green but you will be a very long time getting your money back. However there may come a day when the oil boiler is no longer viable so taking a very long view then a HP now is not the worst idea either.

Countrypaul is correct to say electric heating on E7 and PV is probably overall the cheapest solution

Another apology for my delay in replying to your suggestions and advice, the culprit this time was a faulty router. But there again my original problem could also have been said to be a faulty router!

We almost certainly will go down the road of TS heated by immersion heater(s) from the PV and E7 and also from the existing oil boiler in the old house at least for a trial period including one winter. I did envisage having a spare coil for the possibility of switching to a ASHP if the original set up is not viable. But this would mean, with whatever eventual outcome, we would have a coil that was unused. Could we get by with just one coil and use it either for the oil boiler or a HP? I am aware of the temperature difference of the two systems but does that matter?

Brackwell suggests a separate cylinder for the DHW which of course we already have in the old house 18m away from the outlets in the annexe. Considering a circulating pump to overcome the dead leg this would produce I originally thought this was impossible as there was not a connection in the HWC for the return. Then I discovered the Essex Flange so it is back on. Using the existing HWC would simplify matters considerably. Now all I want from the TS is a coil for either the oil boiler feed or a ASHP, a single immersion heater and connections for the UFH. Simplicity itself.

Another apology for my delay in replying to your suggestions and advice, the culprit this time was a faulty router. But there again my original problem could also have been said to be a faulty router!

We almost certainly will go down the road of TS heated by immersion heater(s) from the PV and E7 and also from the existing oil boiler in the old house at least for a trial period including one winter. I did envisage having a spare coil for the possibility of switching to a ASHP if the original set up is not viable. But this would mean, with whatever eventual outcome, we would have a coil that was unused. Could we get by with just one coil and use it either for the oil boiler or a HP? I am aware of the temperature difference of the two systems but does that matter?

Brackwell suggests a separate cylinder for the DHW which of course we already have in the old house 18m away from the outlets in the annexe. Considering a circulating pump to overcome the dead leg this would produce I originally thought this was impossible as there was not a connection in the HWC for the return. Then I discovered the Essex Flange so it is back on. Using the existing HWC would simplify matters considerably. Now all I want from the TS is a coil for either the oil boiler feed or a ASHP, a single immersion heater and connections for the UFH. Simplicity itself.

Now I think that we are getting somewhere thanks to all your input.

If the coil you have is sized and rated for the heat pump, then there should be no problems transfering heat from the hotter but lower flowing oil burner. It is no good just getting a coil that is rated for say 12KW as if the assumption is that the circulating water is at 70C to transfer 12KW into 20C water then when the circulating water from the HP is at 40C transfering into 20C water you may not get anything like the 12KW no matter what the flow rate is (the limit may be movement of water within the store to take the heat away which will be much lower at a lower temperature differential). Best to make sure the coil will work with both.

A recirculating pump may lose you more energy that you expect, and you almost certainly don't want it running all the time, no point in curculating at 3am (unless you tend to be up all night). Various threads on this forum have discussed recirculating pumps, some have a switch nearby the point of water use that causes the pump to activate thereby ensuring there is hot water 30/60/90 seconds after use others simply have the pump on a timer so there is hot water ready from 07:00 to 08:00 etc. Again worth checking out how much energy the pump will use and how much energy the recirculating water will lose in heat against a simple electrical hot water on demand heater plus electric shower as this will obviously depend on useage which only you can estimate. Since it is a new extension the outlets are likely to be limited via TMVs but worth considering whether supplying water at only say 40C (rather than 60C) would be worthwhile for the reduced heat loss, as this could easily be done with a mixer valve using the cooler water + hot to circulate rather then entirely hot. Also consider that during late autumn, winter and early spring this lost heat from the DHW may contribute to the heating though again how much is in the new extension needs assessing.

Bear in mind that you could use an external flat plate heat exchanger (FPHE) with the thermal store instead of a coil, this might also allow you multiple heat inputs (eg. HP and boiler) with only one set of connections to the tank, but would require and additional pump.

Sorry for the delay. No excuse this time other than a lot of thinking which doesn't come easy!

Thanks for your further suggestions. One of my concerns at the moment is the domestic hot water supply to the annexe via a pumped circulatory set up. Following Countrypaulís suggestion I have done a back of an envelope calculation regarding the insulated pipe heat loss and bronze pump demand and it comes to about 250w or 6kWh per day. Which I is considerable. Part of this would be added to the heating of the annexe but I didn't plan to heat the annexe via a HWC in the old house. I initially considered Countrypaulís example of having a switch to turn on the pump adjacent to the outlet to be a non-starter. There is still that wait although obviously shorter than letting gravity do the trick and probably it would be hotter faster. If the pump is controlled on a time switch then there will be occasions when hot water is required outside the set times. Perhaps combining these two means could be the answer. Hot water would be available instantly at say three two hour periods during the day Ė a little fine tuning would be called for here - and with a slight delay at other times after pressing a button.

At the moment I'm in a bit of a dilemma. I am informed by someone who gave me a quote for just the installation of the UFH that if it is not associated with a renewable energy installation, a ASHP perhaps, then 20% VAT applies on materials and labour. This on the UFH alone would be about £600, then I assume that the installation of the TS and all the additional plumbing and electrical work would also rate for 20% VAT. Is this correct?

Can't the fact that the UFH will largely be supplied by renewable energy by means of the almost 10kWp of PV swing it?

My dilemma is that even the extra VAT payable on the UFH alone would be enough to finance a cheap Chinese ASHP. What have I to loose? I am aware that this is a Navitron site and I can't expect anyone to post agreeable comments on these Chinese ASHPs but you could always email me on: john.uuood@gmail.comBy the way are there any Chinese ASHPs that are accepted on the RHI scheme?

If it is a fact that I will have to pay 20% VAT on everything then a more expensive HP could be paid for with the VAT savings. I need to do a few more calculations, perhaps not on the back of an envelope this time.